Transplantation has become a major therapeutic option for a number of diseases over the past 20 years [Starzl et al., N Engl J Med 320:1014-1021,1092-1099 (1989); TINS 14(8):all pages (1991); Murray, Science 256:1411-1416 (1992)]. In fact, transplantation of many portions of the central nervous system has been achieved in rodents and other species, including animal models of nigrostriatal dysfunction related to Parkinson disease [Lindvall et al., Science 247:574-577 (1990); Goetz et al., New Engl J Med 320:337-341 (1989); Gill and Lund, J Am Med Assoc 261:2674-2676 (1990)].
Gage et al., in U.S. Pat. No. 5,082,670, issued Jan. 21, 1992, discloses the use of genetically modified (by means of retrovirus insertion of genes) fibroblast donor cells for grafting into the central nervous system (CNS) to treat diseased or damaged cells. The fibroblast donor cells can be modified to produce a protein molecule capable of affecting the recovery of cells in the CNS. The entire contents of U.S. Pat. No. 5,082,670 are hereby incorporated by reference into the subject application in order to more fully describe the state of the art of the subject invention.
Another cell which has been transplanted into the CNS is the astrocyte [Zhou et al., J Comp Neurol 292:320-330 (1990)]. Astrocytes have a wide range of functions, including: release of growth and trophic factors; inactivation of neurotransmitters; antigen presentation; ionic regulation; and response to certain lymphokines [Lillien and Raff, Neuron 5:111-1219 (1990); Raff, Science 243:1450-1455 (1989); Kimelberg and Norenberg, Scientific American, pp. 66-76 (April 1989)]. In addition, astrocytes from neonatal and adult sources (including human brain) replicate in vitro. Moreover, unlike fibroblasts, astrocytes belong in the brain and have region specific properties [Shinoda et al., Science 245:415-417 (1989); Batter and Kessler, Molec Brain Res 11:65-69 (1991)]. When transplanted, astrocytes survive at the site of injection and may migrate up to several millimeters into the host brain without forming tumors [Zhou et al. (1990)]. Some of the potential advantages of using astrocytes over skin fibroblasts concern this migration into the host brain, as well as lower epileptogenicity [Jennett, Arch Neurol 30:396-398 (1974)], and their natural expression of neurotransmitter receptors. Furthermore, although inadvertently displaced normal (primary) fibroblasts following spinal taps form spinal fibroma and transplants of established neuronal cell lines (e.g. C6-glioma, PC12 cells, etc.) often form neoplastic tumors, this has not occurred with astrocyte transplantation [Zhou et al. (1990); Emmett et al., Brain Res 447:223-233 (1988)]. Indeed, astrocytes only migrate away with little if any new cell division. In contrast, fibroblasts do not migrate and are limited by a reactive gliosis surrounding the transplant [Kawaja et al., J Comp Neurol 307:695-706 (1991)] while astrocytes can interdigitate between neurons after migration and thus have direct contact with neurons [Zhou et al. (1990)].
In addition to the choice of a particular cell for transplantation, a method for modifying the particular cell must also be chosen. A common method, such as the method disclosed in Gage et al., is viral-mediated gene transfer. Viral-mediated gene transfer raises safety issue problems due to the use of active and potentially pathogenic viruses [Amer Soc for Microbio News 58(2):67-69 (1992)]. For example, the biological properties of retroviruses utilized by Gage et al. have potential for causing mutations or cancer, and the possibility of continued infectivity. Furthermore, the physical dimensions of retroviruses limit the amount of foreign DNA which can be transferred via the retrovirus.
Another alternative method of gene transfer is chemical mediated gene transfer, such as by stable calcium phosphate transfection. The parameters for transfecting cells by this method vary for each different cell type, and therefore need to be determined and optimized for each different cell type.